One of the most challenging problems in modern biology is to determine how gene expression is regulated in higher organisms. The nucleolus, the subnuclear organelle rich in ribonucleoprotein, is the site of ribosomal RNA synthesis. I have demonstrated that nucleoli can be isolated in a form which retain certain of their biological attributes when incubated in vitro, i.e. they are not only capable of incorporating radioactive precursors into RNA, but also reflect physiological (hormonal) modulators of RNA synthesis. Cancer cells are known to manifest aberrant gene expression. Whether this is the cause or a consequence of the neoplastic transformation is at present unclear. This proposal is an attempt to dissect and compare the molecular mechanisms of gene regulation, using ribosomal RNA synthesis in the isolated nucleoli as a model system of normal, hypertrophic, premalignant and neoplastic cells. The biological systems that will be used in this study normal, regenerating liver and livers undergoing neoplastic transformations after treatment with the chemical carcinogens N-OH-2-acetylamino-fluorene and thioacetamide, as well as the already transformed Morris hepatomas of various rates of growth and dedifferentiation. Hormones, such as cortisone, growth hormone, and thyroxine will also be used as additional experimental parameters to probe the intracellular processes which regulate ribosomal RNA synthesis is these various hepatocytes. In addition to in vivo isotopic evaluation of rebosomal RNA synthesis, the degree to which nucleolar DNA template availability or the types or levels of RNA polymerase I alter in responding to the regulatory hormones will be evaluated for each of the above mentioned hepatic systems. Furthermore, the gene products; the ribosomal precursor RNA, will be analyzed in terms of possible size and composition alterations, and the RNA ploymerases, from the above mentioned various sources, will be purified and compared. In all, it is hoped these studies will shed some light on the basic mechanisms of control of gene expression in both normal, premalignant and fully malignant cells.